The invention relates to a system and a method for digital predistortion of a complex baseband input signal which is fed to a transmitted power amplifier to generate a high-frequency multicarrier transmit signal.
With newly introduced radio communication systems, such as the EDGE or the UMTS mobile radio system for example, linear forms of modulation are used in which a constant instantaneous power of a transmit signal is dispensed with. A maximum transmit signal amplitude which arises in a such case typically lies significantly above the average value of an associated transmit power.
In addition there are plans with the newly introduced radio communication systems to use multicarrier data transmission procedures (multicarrier approach) in which a plurality of transmit signals with different carrier frequencies are modulated with a high carrier frequency spacing and fed to a transmit amplifier. A coherent overlaying of all transmit signals in this case means that maximum amplitudes occur which lie far above the average value of the associated transmit power. A transmitter power amplifier must accordingly retain sufficient reserves of power for a linear amplifier operation.
Non-linearities of the transmit power amplifier effect the efficiency of the entire radio communication system. The non-linearities disadvantageously cause intermodulation products of which the carrier frequencies act as interference to disturb the carrier frequencies to be used. These disruptive interferences are suppressed by executing what is referred to as a predistortion of a power amplifier input signal. A distinction is to be made here between an “analog” and a “digital predistortion”, with the digital predistortion offering easier reproducibility and higher flexibility.
A basic block diagram in accordance with the related art is shown in FIG. 13 for example.
A complex digital baseband input signal BEIS is connected both to a device for predistortion PRE of a forwards branch and also to a device for parameter estimation PPRE. In addition to the device for predistortion PRE the forwards branch contains a serial downstream compensation device COM, of a digital-analog converter DAC, a transmit filter causing linear distortions TxF, a modulator MOD and a transmit power amplifier AMP.
A broadband high-frequency multicarrier transmit signal is provided at the output of the transmit power amplifier AMP. The transmit power amplifier AMP is connected on the output side to a demodulator DEM arranged in a feedback branch. The feedback branch features a receive filter RxF following on from the demodulator DEM in series and an analog-digital converter ADC which is connected on its output side with the device for parameter estimation PPRE.
The device for parameter estimation PPRE forms parameters for a predistortion of the baseband input signal BEIS to be performed, with the parameters formed being transmitted to the device for predistortion PRE. With the aid of the compensation device COM linear distortions of the transmit filter TxF, of the modulator MOD and partly of the transmit power amplifier AMP are compensated for.
In the digital predistortion to be performed a restriction of a transmit signal bandwidth is necessary since with the device for predistortion PRE a static non-linearity of the transmit power amplifier AMP is assumed and simulated inverted. For precise implementation of the digital predistortion, taking into account the static non-linearity of the transmit power amplifier AMP at least one reproduction of a third or fifth harmonics of the multicarrier transmit signal is needed, in which case for the D/A converter DAC used at least one triple bandwidth of the multicarrier send signal is needed in the forwards branch.
The same applies to the A/D converter ADC in the feedback branch, to which a broadband predistortion output signal of the device for predistortion PRE is fed via the following modules (COM, DAC, TxF, MOD, AMP, DEM and RxF). For a multicarrier transmit signal with a bandwidth of 60 MHz, corresponding to the UMTS standard, very wide band and cost-intensive A/D converters or D/A converters are thus required.
In this case it should also be stated in conclusion that both the parameter estimation PPRE and also the predistortion PRE are performed with a sampling frequency which is determined by a clock frequency of the D/A converter DAC or of the A/D converter ADC used.